The present invention relates to a method of removing acid gases from fluid streams, in particular a method of removing carbon dioxide from flue gases.
The removal of carbon dioxide from flue gases is desirable for many reasons, in particular, however, for reducing the emission of carbon dioxide which is considered to be the main cause of what is termed the greenhouse effect.
On an industrial scale, use is frequently made of aqueous solutions of organic bases, e.g. alkanolamines, as absorption media for removing acid gases, such as carbon dioxide, from fluid streams. When acid gases are dissolved, ionic products are formed from the base and the acid gas components. The absorption medium can be regenerated by heating, expansion to a lower pressure or by stripping, wherein the ionic products react back to form acid gases and/or the acid gases are stripped off by means of steam. After the regeneration process, the absorption medium can be reused.
Flue gases have very low carbon dioxide partial pressures since they generally occur at a pressure close to atmospheric pressure and typically comprise 3 to 20% by volume of carbon dioxide. In contrast to fluids such as natural gas or synthesis gas, flue gases additionally comprise oxygen. The oxygen also dissolves in small traces in the absorption medium and can lead there, at elevated temperatures, to a degradation of the amine. An absorption medium for separating off carbon dioxide from flue gases should satisfy the following criteria: (i) sufficient capacity at low CO2 partial pressures; (ii) sufficiently rapid absorption rate at low CO2 partial pressures; (iii) stability to oxygen; (iv) low vapor pressure for reducing solvent losses; and (v) low energy requirement for regenerating the absorption medium.
A technology based on monoethanolamine (MEA) is known under the name Fluor Econamine for separating off carbon dioxide from flue gases (cf. e.g. Second National Conference on Carbon Sequestration, National Energy Technology Department of Energy, Alexandria Va., USA, May 5-8, 2003 under the title: Fluor's Econamine FG PlusSM Technology; An Enhanced Amine-Based CO2 Capture Process).
Mixtures of MDEA and piperazine are described in the literature as solvents which are likewise suitable for separating off CO2 from flue gases (Closman, F.; Nguyen, T.; Rochelle, G. T: MDEA/piperazine as a solvent for CO2 capture, GHGT-9, Washington D.C., USA 2008, Nov. 16-20).
Although technologies based on monoethanolamine are distinguished by high reactivity between the amine and carbon dioxide, the high reactivity is disadvantageously accompanied with high absorption enthalpy and thereby a high energy requirement for regeneration. Other alkanolamines such as, for instance, diethanolamine or methyldiethanolamine, which have a lower energy requirement for regeneration, are suitable only with restrictions for this separation task owing to their slower reaction kinetics between carbon dioxide and amine.
EP-A 558019 describes aqueous solutions of sterically hindered amines for removing carbon dioxide from flue gases.
WO 2007/144372 discloses a method of removing carbon dioxide from a gas stream in which the partial pressure of the carbon dioxide in the gas stream is less than 200 mbar, e.g. from flue gas, by contacting it with an aqueous solution of a tertiary aliphatic alkanolamine and an activator, in particular 3-methylaminopropylamine.
WO 2005/087349 describes a method of removing carbon dioxide from a gas stream in which the partial pressure of the carbon dioxide in the gas stream is less than 200 mbar, wherein the gas stream is contacted with a liquid absorption medium which comprises an aqueous solution (A) of an amine compound having at least two tertiary amino groups in the molecule and (B) of an activator which is selected from primary and secondary amines.